PROJECT SUMMARY There is a fundamental gap in our understanding of the immunologic mechanisms leading to BP; therefore, standard treatment remains global immunosuppression, which is associated with significant morbidity and mortality. Our long-term goal is to understand the reason for the failure of peripheral tolerance in BP and identify cellular or molecular targets for therapeutic use. The objective of this application is to determine if the development of BP results from a tissue specific loss of tolerance. Our central hypothesis is that the development of BP lesions is dependent on phenotypic and/or functional alterations in LC and Treg populations in the skin. The rationale for the proposed research is that understanding the factors that influence loss of peripheral tolerance in the skin has the potential to translate into more targeted therapies for organ-specific autoimmune diseases. Thus, the proposed research is relevant to that part of NIH's mission that pertains to expanding the breadth and depth of knowledge of diseases which will enhance health and reduce illness. Guided by our striking preliminary data, this hypothesis will be tested by pursuing two specific aims: 1) To examine the distribution and phenotype of LC and Treg populations in BP skin biopsies; 2) To determine if the loss of tolerance in BP is tissue-specific. Under the first aim, we will expand our preliminary data utilizing confocal microscopy to phenotypically characterize immune cell populations during different stages of the disease. In the second aim, cell populations involved in the maintenance of tolerance will be examined in tissues where tolerance has been lost (skin) or maintained (blood) using flow cytometry to assess cell frequency and phenotype and in vitro functional assays. This project is innovative because it applies the recent advances in our understanding of resident immune cell populations to human autoimmunity in a disease where the target organ, skin, is readily accessible. The proposed research is significant as it will expand our understanding of mechanisms of tissue-specific autoimmunity in humans, which will lead to the development of more targeted therapies.